1. Field of Invention
The present invention relates to measurement of viscosity with a frictionless spring, an arm and a non-contact distance sensor.
2. Description of Prior Art
A liquid between two surfaces will shear when one surface moves relative to the other. The force needed to make such a movement is directly related to the viscosity of the liquid (with the mechanical configuration factored out). Viscometers typically rotate a bob within a cylinder still with the liquid therebetween, or rotate an outer cylinder while keeping the inside coaxial bob inert. In such examples, torque is directly related to the viscosity of the liquid (again with mechanical configuration factored out).
Several types of arrangement have been applied to measure the torque due to the viscosity of the liquid. In U.S. Pat. No. 3,435,666, a helical spring is attached to the inside bob through a bob shaft while driving the outer cylinder. The shear force applied on the bob is proportional to the torque applied by the liquid, which is also measured by a strain gauge torque transducer. One of the drawbacks of this design is that the strain gauge torque transducer is located inside of the pressurized zone. Under elevated temperature and pressure conditions, if pressure is suddenly lost, hot sample could easily boil off and fill the pressurized zone. Thus strain gauge torque transducer would be easily contaminated and damaged by corrosive samples. Also, corrosive sample vapor can go up to the instrument top and damage the strain gauge torque transducer over time as well. Another drawback of this design is that two bearings are required to mount the bob and bob shaft assembly. Those bearings have drag force, which create extra error in the reading. Also, those bearings can be corroded easily under high temperature and corrosive sample vapors. Thus, traditionally it has been a major task to maintain those bob shaft bearings working properly for this type of viscometers. In U.S. Pat. No. 5,535,619, a torque tube is attached to the inside bob while driving the outer cylinder. The toque applied on the bob causes a rotational deflection on the readout wire, which is inside of the said torque tube. The readout wire is in turn mounted on a rigid jewel support located in the instrument head. An electromagnetic sensor pickup the rotations of the readout wire. One of the drawbacks of this design is that the torque tube and readout wire is structurally weak and excessive load can easily damage it. Another drawback of this design is that the lower end of the torque tube and readout wire is located in hot sample zone. High temperature can change the spring property of the torque tube and readout wire. This is the reason that this type of viscometer has poor accuracy and large zero drift at elevated sample temperatures.
It is an object of this invention to provide a viscometer that isolates its electronic torque sensor from corrosive samples and corrosive sample vapors.
It is an object of this invention to provide a reliable, rugged and temperature stable instrument with integrated electronics usable in viscosity measuring applications, under atmospheric, pressurized, low and high temperature conditions.
It is another object of this invention to provide a viscometer that operates with a wide range of liquids with viscoelasticity property measurement capability.
It is another object of this invention to provide a viscometer that eliminates measurement errors due to conventional bearing frictions.
It is another object of this invention to substantially reduce maintenance work yet meets industry standards of accuracy, reliability, durability, dependability, and ease of cleaning.